Recently, small hand-held electronic devices, such as laptop computers, mobile phones, video cameras, digital cameras, portable receivers compatible with One-Seg broadcasting, portable music players, and the like, have become very popular.
Battery packs are employed in those portable electronics for their ease of handling. Typically, a battery pack is formed of one or more secondary batteries or cells assembled in a package, each cell consisting of a high-density battery such as a lithium-ion battery, a lithium polymer battery, and a nickel metal hydride battery. These high-density batteries contain extremely large amounts of energy, so that abnormal voltages in the power circuitry, such as overcharge, overdischarge, or excessive current, may cause the battery pack to generate heat or even to catch fire when overheated.
To avoid heating and ignition and prevent constituent secondary cells from deterioration over time, a battery pack is generally equipped with a charge and discharge protection circuit that protects the power circuitry from abnormal conditions, including overcharge, overdischarge, and overcurrent induced by overcharge and overdischarge, as well as short-circuits, overheating, etc., by shutting off a connection between a secondary battery and a battery charger or load device upon detecting an abnormal voltage across a battery cell.
For example, Japanese Patent Application Laid-Open Publication No. 2008-164567 discloses a battery cell voltage monitoring device, including a voltage sensor to monitor voltages across multiple battery cells, an output logic circuit to output a voltage detection signal indicating detection of abnormal voltage based on the output signal from the voltage sensor, and a delay circuit to provide the output signal with a predetermined delay and forward the delayed signal to the output logic circuit. In this monitoring device, the voltage sensor includes multiple comparators with hysteresis characteristics, each dedicated to one of the battery cells to provide a comparison output based on which the voltage sensor detects a voltage across each specific battery cell.
It is known that a comparator exhibits a varying input offset voltage for a given level of voltage supplied to its power terminal, depending an the level of voltage input to its input terminal for comparison with a reference voltage.
According to the arrangement mentioned above, the voltage sensor comparators are all powered with a voltage output from the series of battery cells as a whole constituting the battery pack, and receive different input voltages from the respective battery cells. Hence, these comparators exhibit different input offset voltages (e.g., a comparator monitoring a cell directly coupled to the supply terminal and a comparator monitoring a cell directly coupled to the ground terminal can have significantly different input offset voltages), and can detect different levels of abnormal voltages for a common reference voltage even where they are substantially identical in configuration. This affects precision of voltage detection in the conventional battery cell voltage monitoring device.
Another problem encountered by conventional battery packs is the difficulty in designing a protection circuit that can be used in various types of battery packs regardless of the number of secondary cells used in the battery pack.
As mentioned, a battery pack consists of a set of two or more battery cells connected in series. The number of such cells differs by the type of equipment in which the battery pack is installed, e.g., about 2 to 4 cells in the case of those used for small, hand-held devices. On the other hand, a protection circuit is provided as a semiconductor device integrated into an integrated circuit (IC), which has a specific number of voltage detectors or detection terminals each dedicated to a single battery cell in a multi-cell battery pack.
Thus, manufacturers of protection ICs are required to provide products with different numbers of detector elements for different battery packs depending on the number of secondary cells. However, producing a variety of products identical in operation for different types of battery packs is inefficient and costly.
To avoid such inefficiency, it is possible to use a protection circuit in a battery pack by short-circuiting terminals of detectors out of use in the protection circuit. This method will work where the number of secondary cells in the battery pack is less than the number of voltage detectors in the protection circuit (e.g., a protection circuit with four voltage detectors can be used with a battery pack containing 2 or 3 secondary cells). However, short-circuiting the unused detector terminals can result in malfunctioning of the protection circuit, where the voltage detector senses the short-circuit as an overdischarge to turn off a switch transistor connected between the battery and load circuitry.
Currently, there are several protection IC products commercially available that can be used with different numbers of battery cells. For example, one such battery protector, which is compatible with both four-cell and three-cell batteries, includes four voltage detection circuits for four battery cells. Each of the four detection circuits is provided with a comparator for detecting overdischarge of a specific one of the battery cells, as well as a selection terminal connected to an external signal source, which forces the output of a predetermined one of the four comparators to go inactive upon receiving a negation signal. Thus, supplying the negation signal makes the circuit for use in a three-cell battery, and not doing so holds it in the original state which is suitable for use in a four-cell battery.
However, such a method has limitations in that implementing the cell number selection capability in existing protection circuitry will require a lot of modification to the circuitry, involving changes in the configuration of the voltage detection comparator. Such modification can be further complicated when the protection circuit is configured to provide selection capability for a wide range of battery cell configurations.